Here is the full list of all 21 Rubber Ducky and the Rainbow Gun achievements.
You've defeated the derpy brother and unlocked a message from the developer. . .
Get below 8:00 in speed run mode.
Place fifty-jillion (100) plants in the bounce house at once.
Play with Death Epileptic Pixel mode on. (There's also a game of the same name)
Beat the dev's score on the last level! (You might want to shoot for around 1500)
Convince/bother/bribe the dev until everyone gets this achievement.
Type the 2nd section of the dialog from the 6th level backwards into the cheat code selector! Don't forget to puncuate and capitalize properly!
Be addicted to social media.
Unlock all the levels!
. . . or it would have been, anyway!
I told you to go right, didn't I. . .?
Buy all the costumes!
Everyone knows it's the best part!
It's there, it's unique! Just look closely!
Well, it's sorta' in the level. . .
This one's not too hard to find. . .
Have the game run below 30 Frames Per Second.
Looks like someone just likes firing the Rainbow Gun. . .
Well, it's still there. . . just less dire.
It's a practical paradise, just for you!
You're the best! :D